THIS invention relates to an electrochemical cell, in particular a lead-acid battery cell, and to an additive for reducing water loss thereof.
A problem inherent in lead-acid battery cell usage is that it is difficult to provide a charging current having a magnitude that exactly matches the requirement of the cell. It is, therefore, customary to apply a degree of overcharging to ensure that the cell is not inadvertently short charged. It is known that such overcharging causes electrolysis, and consequently, that this leads to a loss of water from the electrolyte.
It is usual for the electrolyte to contain between 30% and 40% of sulphuric acid, by weight, in water, when the cell is in a full state of charge, and roughly half this concentration of acid when the cell is depleted.
At full state of charge of the cell the negative electrode includes an active portion of lead and the positive electrodes includes an active portion of lead dioxide.
Upon discharge, the active portion of lead in the negative electrode and the active portion of lead dioxide in the positive electrode are each converted to lead sulphate.
The relevant reactions come into effect when the electrical circuit connected to the cell is completed and an electric current is permitted to flow therein. The cell is recharged simply by causing the current in the circuit to reverse, thereby causing the reactions to be reversed. The reactions will slow down as the cell becomes fully charged, and cease upon attainment of a full state of charge.
The customary overcharged which ensues is made as small as possible within practical limitations, and especially so in the case of sealed batteries where the avoidance of water loss is even more important. However, once the reactions between the electrolyte and the electrodes have finally ceased, the current being applied to the cell electrolyses the water portion of the electrolyte into its primary constituents, hydrogen gas at the negative electrode and oxygen gas at the positive electrode, effecting a loss of water. The functional chemistry of the electrodes is substantially unaffected by this process of electrolysis, which affects principally the electrolyte through decomposition.
U.S. Pat. No. 3,928,066 to Lewenstein discloses a rechargeable lead acid storage battery incorporated a quaternary ammonium compound wherein aromatic and/or aliphatic groups have been substituted for all the hydrogen atoms for the purpose of suppressing hydrogen evolution and thereby inhibiting water loss.
A disadvantage of the Lewenstein invention is that it is limited to providing suppression of gas evolution passively through minimising the latent chemical reactions between the electrolyte and antimony-to-lead couples existing in one of the electrodes, also known as local action, and does not provide for suppression of gas evolution caused by electrical stimulation provided by an external circuit.
Another disadvantage is that quaternary ammonium compounds have a limited life in a lead-acid battery even if the cell remains unused. A dosage, whether 200 ppm or 2 percent in concentration initially, can fall to near zero within a few weeks, and consequently provide the claimed benefit for perhaps less than ten percent of the service life of the cell.
It is an object of the present invention to overcome these and other disadvantages by providing means for suppressing the evolution of gas from an electrochemical cell while an electrode of the cell is in a state of charge, particularly in a so-called “overcharged” state.